Woah, here we go again! It was great to see the information taken from the Tyvek website. The only thing you're all missing is that where there's liquid water, there's water vapor. PERIOD. Laws of physics, gentlemen. You get water back there, sometime later the water has "dried up". Can you say 'phase change" it simply doesn't disappear, it goes from a liquid to a vapor and where the vapor goes is dependent on on about 2 zillion variables, most of which will bite you in the *** over time. You can all probably search my posts and the endless **** I took for my distaste of Tyvek and other spunbonded olefin house wraps. Let it suffice to say I have made plenty of good weeks pay ripping out rotted sheathing, punky studs and soaked insulation,all because the original contractor (myself included) thought the stuff was Gods gift to the building trade.
One more comment to anyone who thinks vinyl siding is a weather tight assembly... Take a hose to a vinyl house or wait for your basic driving rainstorm. Zip a few courses of the **** off the building and tell me its dry back there